1. Field of the Invention
The present invention relates to a polarization scrambler for scrambling the polarization of signal light, and in particular to a polarization scrambler wherein the management for example of an incident angle of a polarization direction of incident signal light is facilitated.
2. Description of the Related Art
Recently there has been progress with increases in speed and increase in long distance transmission in optical communication systems, with one of the key devices for realizing this being the polarization scrambler.
Presently long distance transmission in optical communication systems is carried out for example with a large number of optical fiber amplifiers connected in multiple stages. With such optical transmission systems, noise light generated by the optical fiber amplifiers depends on the polarized state of the signal light and hence a phenomena where excessive noise light increases (polarization hole burning) has been pointed out. Since due to the polarization hole burning, the mean value of the signal light to noise light ratio (optical SNR) decreases while the fluctuation of the optical SNR increases, this becomes a problem in performing long distance transmission. Furthermore, in the case of wavelength division multiplexing (WDM) transmission systems which multiplex and transmit light of several wavelengths, four-wave mixing (FWM), being one of the nonlinear effects of the optical transmission path, is produced so that transmission characteristics are degraded. With four-wave mixing, it is known that the generation efficiency thereof becomes a maximum in the case where the polarization states of several signal lights coincide.
As a technique for reducing the influence of such polarization hole burning and four-wave mixing, for example a technique such as polarization scrambling which positively changes the polarization of the signal light on the transmitting side to give a non polarized state, has been proposed. One means for making the polarization of the signal light random is a polarization scrambler.
As conventional polarization scramblers there are for example the known devices of Japanese Unexamined Patent Publication Nos. 9-162809, 9-211391, and 9-113854. As an example of such conventional polarization scramblers, there is the device shown in FIG. 10 which uses an electro-optic effect.
The polarization scrambler of FIG. 10 is one where an optical waveguide is formed on a substrate such as lithium niobate (LiNbO.sub.3 : referred to here under as LN) having an electro-optic effect, an electrode is provided on the substrate, and an optical fiber is optically coupled to the input and output side of the optical waveguide. With a polarization scrambler of this construction, as shown in FIG. 10(b), a signal light of linear polarization is input with the polarization direction at an incline of 45.degree. to the vertical direction of the optical waveguide, and the linear polarization is resolved into a vertical component and a horizontal component. At this time, by applying a modulation signal of for example a sinusoidal wave to the electrode provided on the optical waveguide, the refractive indexes for the vertical component and the horizontal component of the optical waveguide change due to the electro-optic effect so that the speeds of the respective direction components propagated inside the optical waveguide change. As a result, a phase difference is produced between the vertical component and the horizontal component of the signal light, and as shown in FIG. 10(c) the polarization state for the output signal light becomes random.
Incidentally, one important characteristic for a polarization scrambler is the degree of polarization (DOP). The degree of polarization is a parameter showing the extent of polarization. That is to say, if the strength of the optical component in the fully polarized state is Ip, and the strength of the optical component which is not in the polarized state is Im, then the degree of polarization P is expressed by the following equation (1) which gives ratio of the polarized light component with respect to the total light amount. EQU P=Ip/(Ip+Im) (1)
More specifically, this shows that when the value for the degree of polarization is close to zero, polarization is approximately random, while when close to 100%, polarization is approximately constant.
With the conventional polarization scrambler which utilizes the above electro-optic effect, the degree of polarization depends on the voltage of the modulation signal applied to the electrode or on the incident angle of the polarization direction of the signal light of the linear polarization input to the optical waveguide. Controlling the voltage of the signal applied to the electrode to minimize the DOP can be achieved relatively easily. However, accurately managing the incident angle of the signal light is not so easy. Heretofore, when an optical fiber is fixed to the input side of the polarization scrambler, the incident angle of the polarization direction of the signal light output from the optical fiber is adjusted so as to become 45.degree. .+-.0.5.degree. with respect to the vertical direction of the optical waveguide, to minimize the degree of polarization. However such adjustment is extremely difficult and takes much time so that there is the problem of poor manufacturability. Furthermore, once the optical fiber is fixed to the polarization scrambler, subsequent incident angle adjustment is not possible. Therefore there is also the problem that for example, it is difficult to cope with the situation when the degree of polarization changes due for example to degradation with time of the optical system components.